1. Field of the Invention
The present invention relates to a printer head of a printer that prints onto recording paper and, more particularly, to an impact type dot printer head that operates on a pixel printing method.
2. Discussion of the Background
FIG. 3 illustrates a typical conventional dot printer head. In FIG. 3, reference numeral 1 is a housing that has an opening 2 on one side thereof. The housing 1 contains a base 3 and a plurality of electromagnets 4 arranged in circle. The electromagnets 4 comprise a circular yoke 5, cores 6 integrally formed inside the yoke 5, coils 7 that surround the cores 6, and armatures 9 that hinge on the edge of the yoke 5 as a fulcrum 8 and are thereby supported detachably against the cores 6.
The back ends of needles 10 are brazed to the free ends of the armatures 9. The tips of the needles 10 are slidingly supported by needle guides 11, 12 and 13 fixed to a nose 14. The nose 14 is integrally formed in the housing 1. The needle guide 11, which is the closest to the opening 2 of the housing 1, contains springs 15 that forcibly detach the armatures 9 from the cores 6.
A cover 16 is attached with screws 21 to an opening edge la of the housing 1, closing the opening of the housing. Fulcrum pressure springs 17 are attached to the cover 16. These springs push the base of the armatures 9 against the yoke 5. An armature stopper 18 is fixedly mounted behind the armatures 9, the stopper 18 coming into contact with the rebounding tips of the armatures 9.
The tip of the nose 14 is located opposite to a platen 22. Recording paper 23 and an ink ribbon 24 are threaded between the tip of the nose 14 and the platen 22.
In the above setup, when a specific coil 7 is energized, the corresponding armature 9 is attracted to the corresponding core 6. This causes the needle 10 to strike the platen via the ink ribbon 24 and recording paper 23, printing a dot on the paper. Immediately after this, the armature 9 rebounds thanks to the spring 15. At this point, the armature 9 comes into contact with the armature stopper 18 and stops there.
The dot printer head of FIG. 3 has each needle 10 fixed to the tip of each armature 9. Because the needles 10 must unfailingly strike the platen 22 via the ink ribbon 24 and recording paper 23, the head structure requires its components to be positioned in such a way that each armature 9 does not contact the corresponding core 6 when attracted to the latter. As a result, the stroke of the needles 10 is held small and the gap between the tip of each needle 10 and the platen 22 is minimized.
The above-described prior art dot printer head has one distinct disadvantage. That is, the armature stopper 18 is made of rubber or like elastic substance to absorb the rebound energy of the armatures 9 and thus has a thermal expansion coefficient greater than those of metals. As a result, during an extended printing operation, the heat from the coils 7 expands the armature stopper 18 in thickness. The expanded armature stopper 18 pushes the armatures 9 in the printing direction, causing the tips of the needles 10 to protrude beyond the needle guide 13. If the printing operation is continued in this state, the actions of the armatures 9 and needles 10 fail to follow the frequency of print pulses applied to the electromagnets 4, i.e., the printing frequency. This can lead to degraded printing quality and/or to a mechanical failure such as the ink ribbon 24 getting caught by a tip of a needle 10.
One prior art solution to the above problem involves providing the housing 1 and/or the cover 16 with a large number of radiating fins 25 for higher efficiency in heat radiation. In this case, the shape of the housing 1 and/or the cover 16 tends to be complex and the cost thereof is correspondingly higher. Another prior art solution is the use of a temperature sensor which, as the temperature in the housing rises, reduces the printing speed of the printer. One disadvantage of this solution is higher cost; another is lower printing efficiency.